A known internal combustion engine permits use of a plurality of types of fuels, each having properties different from each other. An example of such an internal combustion engine is one which uses ethanol-blended gasoline. When fuels with properties different from each other are to be used, it becomes necessary to adjust an air-fuel ratio according to the fuel properties. When, for example, the ethanol-blended gasoline is to be used, the air-fuel ratio must be adjusted according to ethanol concentration of the fuel, because ethanol and gasoline have heat values per unit volume that are widely different from each other.
Properties of a fuel used are not necessarily well-known and do not remain constant at all times. Take the ethanol-blended gasoline, for instance, the ethanol-blended gasoline is commercially available in a plurality of types, each having a unique ethanol concentration. A fuel of a first ethanol concentration may therefore be added to a fuel having a second ethanol concentration in the fuel tank. An internal combustion engine, in which a plurality of types of fuels, each having properties different from each other, may be used, therefore needs means of identifying properties of the fuel being used therein.
Fuel property sensors have been used as the abovementioned means. Techniques relating to the fuel property sensors are disclosed in, for example, JP-A-2008-157728 and JP-A-5-045281. The fuel property sensors disclosed in these Patent Documents are an optical type. The fuel property sensors of other types are also known, such as that disclosed, for example, in JP-A-2008-014741. Most of the fuel property sensors include a sensor element and a circuit, though operating on different detecting systems. Electricity must therefore be supplied to operate the fuel property sensors.
Operating the fuel property sensor allows the properties of the fuel used in the internal combustion engine to be determined. Operating the fuel property sensor, however, consumes electric power and, moreover, consumes power that is far from little. In addition, the fuel property sensor is controlled to be activated or deactivated by a vehicle electronic control unit (ECU). This means that the ECU also consumes power while the fuel property sensor is being operated. From a viewpoint of load on the ECU, and from a viewpoint of fuel economy of the vehicle, power consumed to operate each device should preferably be kept to a minimum. The fuel property sensor is not an exception to such a need and there is a need for determining properties of fuels with as small power consumption as possible.